Translational research in oncology: the need of additional in vitro preclinical testing methods for new drugs

Cancer remains one of the leading causes of death in the Western world. Despite bold advances in therapeutic oncology, new drug development is infamously ineffective due to the lack of predictive in vitro models. Most patients that suffer from cancer do not die from the primary tumor but due to the development of metastases. And yet current in vitro screening methods for new drugs in oncology still largely target cytotoxicity or the inhibition of cell growth, in which a potential anti-metastatic activity cannot be assessed. Herein the current in vitro models in oncology are reviewed and a new rationale for the pre-clinical development of specific, anti-metastatic therapeutic agents is introduced.

Connecting a tumor to the environment

Tumor cells are not only susceptible to signals from the environment, but they likewise release signal substances. It is well known that tumor cells secrete angiogenic factors--most prominently the vascular endothelial growth factor--which initiate the vascularization of the tumor for its nourishment. This process has been termed neoangiogenesis. Besides this, two further processes have recently been discovered that facilitate the interaction of the tumor with the lymphatic system and the nervous system, named lymphangiogenesis and neoneurogenesis. These three "geneses" have a cognate, in part common regulation and conjointly promote metastasis development. Neoangiogenesis and lymphangiogenesis provide the structures for the two routes of tumor cell dissemination, i.e. either hematogenous or lymphatic. Neoneurogenesis accomplishes the innervation of the tumor by the ingrowth of nerve endings into the tumor and alternatively or additionally by the protection of existing nerve cells from destruction. These tumor-innervating nerve cells may release neurotransmitters which are proliferative or promigratory signals for the tumor cells. Furthermore, nerve fibers are used as routes for tumor cell dissemination, too, which is known as perineural invasion.

Stem cell migration: a quintessential stepping stone to successful therapy

Migration is an innate and fundamental cellular function that enables hematopoietic stem cells (HSCs) and endothelial progenitors (EPCs) to leave the bone marrow, relocate to distant tissue, and to return to the bone marrow. An increasing number of studies demonstrate the widening scope of the therapeutic potential of both HSCs and endothelial cells. Therapeutic success however not only relies upon their ability to repair damaged tissue, but is also fundamentally dependent on the migration to these areas. Extensive in vivo and in vitro research efforts have shown that the most significant effects seen on HSC migration are initiated by the chemokine SDF-1alpha. In this review we will elucidate the many cellular and systemic factors of HSC and EPC cell migration and their modi operandi.

The norepinephrine-driven metastasis development of PC-3 human prostate cancer cells in BALB/c nude mice is inhibited by beta-blockers

The development of metastases is a decisive step in the course of a cancer disease. The detection of metastases in cancer patients is correlated with a poor prognosis, and over 90% of all deaths from cancer are not due to the primary tumor, which often can be successfully treated, but are due to the metastases. Tumor cell migration, a prerequisite for metastasis development, is not merely genetically determined, but is distinctly regulated by signal substances of the environment including chemokines and neurotransmitters. We have shown previously that the migration of breast, prostate, and colon carcinoma cells is enhanced by the stress-related neurotransmitter norepinephrine in vitro, and that this effect can be inhibited by the beta-blocker propranolol. We now provide for the first time evidence for the in vivo relevance of this neurotransmitter-driven regulation using PC-3 prostate carcinoma cells. The development of lumbar lymph node metastases in athymic BALB/c nude mice increased with the application of norepinephrine via microosmotic pumps, while propranolol inhibited this effect. However, the growth of the primary tumor was not affected by either treatment. Additionally, experiments using human tissue microarrays showed that 70-90 percent of breast, colon, and prostate carcinoma tissues express the relevant beta2-adrenoceptor. Thus, our work contributes to the understanding of the basic cellular mechanisms of metastasis development, and furthermore delivers a rationale for the chemopreventive use of clinically established beta-blockers for the inhibition of metastases.

Neoneurogenesis: Tumors may initiate their own innervation by the release of neurotrophic factors in analogy to lymphangiogenesis and neoangiogenesis

Malignant tumors frequently release angiogenic factors, which lead to the vascularization of the tumor, a process called neoangiogenesis. This neoangiogenesis provides sufficient nourishment of the tumor when it exceeds a certain size. Recently, a similar mechanism has been postulated for the development of new lymph vessels in tumors, termed lymphangiogenesis. Thus, tumors get access to the circulation and lymph drainage like any other growing or regenerating tissue. Furthermore, it has been hypothesized that neoangiogenesis and lymphangiogenesis support metastasis development. Elaborating on this model, we herein present strong arguments for the new theory that tumors initiate their own innervation by the release of neurotrophic factors in analogy to lymphangiogenesis and neoangiogenesis. For this process, we coin the term neoneurogenesis. It is likely that neoneurogenesis further supports the formation of metastases, since the ingrown nerve endings can release neurotransmitters which enhance the metastasis development. Strikingly, the presence of nerve cell markers in tumor tissues has been shown to be a prognostic marker for the course of a cancer disease, and we have recently reported on the metastasis-increasing function of the neurotransmitter norepinephrine in a mouse model.

Analysis methods of human cell migration

The autonomous migration of specialized cells is an essential characteristic in both physiological and pathological functions in the adult human organism. Leukocytes, fibroblasts, and stem cells, but also tumor cells, are thus the subject of intense investigation in a broad range of research fields. A wide spectrum of methods have therefore been established to analyze chemokinetic and chemotactic cell migration, ranging from easy-to-handle two-dimensional surface migration assays to highly specialized three-dimensional and intravital analysis methods. It is now manifest that the results obtained with these various migration assays substantially differ. This review therefore gives an overview of the migration assays which are currently in use, describes the methods, and critically enlightens the particular advantages and disadvantages of each method.

Neurotransmitters and Chemokines Regulate Tumor Cell Migration: Potential for a New Pharmacological Approach to Inhibit Invasion and Metastasis Development

The migration of tumor cells is a prerequisite for tumor cell invasion and metastasis development, which accounts for over 90% of cancer mortality. Therefore a major focus of current tumor biological research is the study of those factors that regulate tumor cell migration. Those chemokines and neurotransmitters that bind to G-protein coupled receptors (also known as serpentine receptors) are the most prominent of these factors. Neurotransmitters have been identified that have not only a stimulatory (e.g. norepinephrine) effect, but an inhibitory effect (e.g. GABA) as well. This is an especially fortuitous development, because many known agonists and antagonists of neurotransmitter receptors are currently being successfully used in the treatment of other pathological conditions (e.g. beta-blockers in the treatment of cardiovascular diseases). Likewise, chemokine receptor antagonists, which are under development for the treatment of HIV or rheumatoid arthritis, may be effective tools for the inhibition of chemokine-driven tumor cell migration as well. A further approach to inhibit tumor cell migration arises from the investigation of the relevant signal transduction pathways. The PKC alpha, for example, is a key enzyme in the regulation of tumor cell migration, but not of leukocyte migration. It thus offers a selective target opportunity for specific pharmacological agents to interfere with tumor cell migration. In this review we therefore summarize the current findings on those serpentine receptors involved in the neurotransmitter- and chemokine-regulated tumor cell migration, on the underlying signal transduction pathways, and on the opportunities to inhibit tumor cell migration and ultimately metastasis development with pharmaceutical agents.

Induction of a metastatogenic tumor cell type by neurotransmitters and its pharmacological inhibition by established drugs

The active migration of tumor cells, a crucial requirement for metastasis development and cancer progression, is regulated by signal substances including neurotransmitters. We investigated the migration of tumor cells within a three-dimensional collagen matrix using time-lapse videomicroscopy and computer-assisted analysis of the migration path. Tumor cell migration is induced by norepinephrine, dopamine and substance P. We show that this induced migration, using MDA-MB-468 breast and PC-3 prostate carcinoma cells, can be inhibited by using specific, clinically established receptor antagonists to the beta2-adrenoceptor, the D2 receptor, or the neurokinin-1 receptor, respectively. All of the investigated neurotransmitters significantly activated the cyclic adenosine-monophosphate response element binding protein (CREB). Furthermore, microarray analysis revealed changes of gene expression toward a highly motile tumor cell type, including an upregulation of the alpha2 integrin, which is an essential adhesion receptor for collagen in migration. The gene for the tumor suppressor gelsolin was downregulated. These 2 critical alterations were confirmed on the protein level by flow-cytometry and immunoblotting, respectively. Neurotransmitters thus induce a metastatogenic tumor cell type by directly regulating gene expression and increased migratory activity, which can be prevented by established neurotransmitter antagonists.

Tumor cell locomotion: differential dynamics of spontaneous and induced migration in a 3D collagen matrix

Although great strides have recently been made in elucidating the factors initiating tumor cell migration and the relevant cellular pathways involved, the constituent components of migratory dynamics for individual tumor cell motion have still not been resolved. Utilizing a three-dimensional (3D) collagen assay and computer-assisted, continuous single cell tracking, we investigated the basic parameters for both the spontaneous and norepinephrine-induced migration of highly metastatic MBA-MB-468 breast, PC-3 prostate, and SW 480 colon carcinoma cells. We show that tumor cells do not migrate with uniform migrational structure and speed as previously thought, but rather, the induction of locomotion elicits significant increases in speed, break frequency, and total cell displacement, but decreases in break length and no change in the recruitment of nonlocomotory cells. We furthermore illustrate the corresponding morphological changes of induced tumor cell migration with emphasis on motion in a collagen matrix. These results demonstrate the complexity of tumor cell migration, and the compulsion for incorporating not only knowledge of intracellular pathways, but also fundamental parameters of migratory behavior into any expansive theory of tumor cell migration and metastasis formation. We furthermore establish the analytical methodology of investigating both the stimulation and potential pharmaceutical inhibition of tumor cell migration.

Tumour-cell migration, invasion, and metastasis: navigation by neurotransmitters

Cancer starts as a localised disease, which, if detected early, can often be treated successfully by removal of the primary tumour. A pernicious progression is the invasion of tumour cells into surrounding tissues, resulting in development of distant metastases. Because active migration of tumour cells is a prerequisite for tumour-cell invasion and metastasis, a pressing goal in tumour biology has been the elucidation of factors regulating the migratory activity of these cells. The most prominent regulatory factors are ligands to serpentine receptors-eg, chemokines and neurotransmitters. Many types of neurotransmitter receptors are expressed on tumour cells, supporting the theory that psychosocial factors are involved in the progression of cancer. Understanding how such receptors regulate migration and the availability of specific receptor antagonists could open up new avenues for chemoprevention of tumour-cell migration and metastatic development.

Neurotransmitters regulate the migration and cytotoxicity in natural killer cells

Natural killer (NK) cells and cytotoxic T lymphocytes (CTL), the functional coordination of which are governed by various signal substances, are crucial in the body's defense of tumor and virus-infected cells. We investigated the role of various neurotransmitters and hormones on the regulation of functional parameters, including NK cell cytotoxicity, and the migration of NK cells and CTL within a three-dimensional collagen lattice. Using peripheral blood CTL and NK cells, we show that the neurotransmitters endorphin, histamine and substance P increase NK cell cytotoxicity, while norepinephrine inhibits cytotoxicity. Moreover, substance P reduces migratory activity, while norepinephrine increases NK cell and CTL migration. Furthermore, all three steroid hormones which were investigated, namely cortisone, testosterone, and estradiol, had regulatory influence on both cytotoxicity and migration of NK cells. These results further specify the functional basis of the complex interconnection between the immune and neuro-endocrine systems.

Expression, Isolation, and Crystallization of the Catalytic Domain of CopB, a Putative Copper Transporting ATPase from the Thermoacidophilic Archaeon Sulfolobus solfataricus

The P-type CPX-ATPases are responsible for the transport of heavy metal ions in archaea, bacteria, and eukaryotes. We have chosen one of the two CPX-ATPases of the thermophile Sulfolobus solfataricus, CopB (= SSO2896) for the investigation of the molecular mechanism of this integral membrane protein. We recombinately expressed three different soluble domains of this protein (named CopB-A, CopB-B, and CopB-C) in Escherichia coli and purified them to homogeneity. 3D crystals of CopB-B, the 29 kDa catalytic ATP binding/phosphorylation domain were produced, which diffracted to a resolution of 2.2 A. CopB-B has heavy metal stimulated phosphatase activity, which was half maximal in the presence of 80 microM Cu2+. The protein forms a phosphorylated intermediate with the substrate gamma-(32P)-ATP. No specific activation of the polypeptide was observed, when CopB-B phosphatase activity was tested in the presence of the purified CopB-C and CopB-A proteins, which provide the cation binding and the phosphatase domains. We conclude that CopB is a putatively copper translocating ATPase, in which structural elements integrally located in the membrane are required for full, coordinated activation of the catalytic ATP binding domain.

Effects of neurotransmitters on the chemokinesis and chemotaxis of MDA-MB-468 human breast carcinoma cells

Most patients suffering from breast carcinoma do not die due to the primary tumor but from the development of metastases. Active migration of cancer cells is a prerequisite for development of these metastases. We used time-lapse videomicroscopy and computer-assisted cell tracking of MDA-MB-468 human breast carcinoma cells, which were incorporated into a three-dimensional collagen matrix, in order to analyze the migratory activity of these cells in response to different neurotransmitters. Our results show that met-enkephalin, substance P, bombesin, dopamine, and norepinephrine have a stimulatory effect on the migration of the breast cancer cells; moreover, these cells show positive chemotaxis towards norepinephrine as was analyzed by the directionality and persistence on a single-cell basis. Gamma-aminobutyric acid (GABA) however has an inhibitory effect. Endorphin and leu-enkephalin, as well as histamin and acetylcholine, had no influence on the migratory activity of the cells. In summary, we provide evidence for a strong regulatory involvement of neurotransmitters in the regulation of breast cancer cell migration, which might provide the basis for the use of the pharmacological agonists and antagonists for the chemopreventive inhibition of metastasis development.

Neurotransmitters are regulators for the migration of tumor cells and leukocytes

Neurotransmitters are signal substances that have traditionally been regarded as mere mediators of signal states between cells in the nervous system. Whereas the mechanisms of this "classic" neurotransmitter regulation are well understood, only recently has new evidence come to light elucidating the modulatory role of neurotransmitters in immune function, and in the regulation of migration of leukocytes and tumor cells. The migration of leukocytes is, among other things, of primary importance for an anti-tumor immune response, whereas the migration of tumor cells is a prerequisite for invasion and the development of metastases. We here clarify and consolidate the latest tumor biological findings on the role of these neurotransmitters, which bind to serpentine receptors, and which are involved in leukocyte migration, tumor growth, invasion and metastasis. This review thus accentuates the complex, interactive involvement of neurotransmitters in the regulation of migration of both leukocytes and tumor cells.